1. FIELD OF THE INVENTION
The present invention relates to a contact type pressure detecting apparatus and a pressure propagating speed detecting apparatus to which the pressure detecting apparatus is applied.
2. RELATED ART
A pressure detecting apparatus (hereinafter referred to as a pressure sensor) used in, for example, an apparatus for sphygmic diagnosis based upon vascular pulses in oriental medicine is adapted to directly bias the artery for detecting the arterial pulses.
The sphygmic diagnosis in the oriental medicine will be briefly described.
In the oriental medicine, the conditions of a patient are determined by sphygmic diagnosis solely relying on the sense of touch of the arterial pulses on the "sunko", that is, the processus styloideus radii in the inner side of the wrist. In the sphygmic diagnosis of the oriental medicine, the pulses on the "sunko" are classified into those on three spots, such as upper, middle and lower spots which are referred to as "shun", "khan" and "shaku", respectively and two kinds of pulsation condition "myakki" on the pulse route "keimyaku" appeared on respective spots are sensed.
The term "shun" means the distal end side of the artery of the wrist. The pulses on the "shun" represent the health conditions of a patient from the head to the chest. The term "khan" means the middle artery of the wrist between the distal end and the heart. The pulses on the "khan" represents the health conditions between the chest and the navel. The term "shaku" means the heart side of the artery in the wrist and the pulses on the "shaku" represents the health conditions between the navel and the toe.
A sphygmic diagnosis apparatus in which arterial pulses are detected from the artery of a human being by means of sensors such as infra-red ray sensor or pressure sensor for performing the diagnosis by observation of the sphygmogram has heretofore been known as is disclosed in the specification of the Japanese Examined Patent Publication No. 57-52054.
The disclosed diagnosis apparatus comprises three pressure sensors 51, 52 and 53 for converting the arterial pulses on the three spots such as "shun" "khan" and "shaku" of the "sunko" into electrical signal waves and a cuff band 55 which is mounted on the wrist 54 of a patient for biasing the pressure sensors 51, 52 and 53 upon the artery of the wrist as shown in FIG. 4.
The pressure sensors 51, 52 and 53 are disposed on the wrist 54, that is, on and along the artery in the "sunko" and the cuff 55 is wrapped around the wrist. A compressed air is pumped into an air bag (not shown) provided on the cuff 55 from a pneumatic pump via a conduit 56. The arterial pulses can be measured by adjusting the amount of the pumped air to change the pressure applied upon the artery. The pressure sensors 51, 52 and 53 are connected with an electromagnetic oscillograph and the like through connection codes 51, 52 and 53, respectively so that the measured arterial pulses are recorded on a recording paper and the like for observing the sphygmogram.
The pressure sensors 51, 52 and 53 comprises, for example, so-called electrostatic microphones or piezoelectric microphones. Specifically, in case of the electrostatic microphone, a high d.c. voltage is applied across an electrode of a vibrating plate and a fixed electrode via a resistor of several tens M.OMEGA.. The electrode of the vibrating plate is brought into direct contact with a spot on which a pressure is detected, for example the artery of the "sunko". The spacing between the electrode of the vibrating plate and the fixed electrode is changed due to pressure to change the electrostatic capacity therebetween. The voltage generated at this time is detected.
The pressure sensing surface of the pressure sensor used for, for example, the sphygmic diagnosis apparatus requires an enough rigidity since the vibrating plate electrode upon which pressure is applied, that is, the pressure sensing surface is biased toward the artery. Such a pressure sensor will be hereinafter referred to as contact type pressure sensor. Accordingly, the contact type pressure sensor 60 is formed so that a pressure sensing surface 61 forms a part of a housing as shown in, for example, FIG. 5B. The contact type pressure sensor 60 thus has a sensitivity distribution in which the sensitivity is the highest in the center of the pressure sensing surface 61 and decreases in a direction toward the outer periphery thereof.
Therefore, when the pressure on the artery of the "sunko" which is smaller than the pressure sensing surface of the contact type pressure sensor, if the pressure sensing surface of the contact type pressure sensor is differently positioned relative to the spot to be detected, a largely different error is resulted so that reproducibility of measuring is not good.
A method of measuring the speed of a fluid flowing through a tube having an elasticity, for example, the speed of blood flowing through the artery and the pressure propagating speed may include two contact type pressure sensors biased toward the artery for determining the speed based upon the distance between the pressure sensors and the pressure propagating time (speed=distance/time).
In this case, it is necessary to adjust the areas of the pressure sensing surfaces of the pressure sensors depending upon the speed of the fluid to be measured. For example, the wave form of the pressure detected by the pressure sensor is changed so that the half value width becomes narrower as the peak value increases when the speed of the fluid is high and the half value width becomes wider as the peak value decreases when the speed of the fluid is low. It is necessary to detect the pressure at a small area of the pressure detecting surface (hereinafter referred to as pin point) when the speed is low like a blood flow. However, the pin point detection has a problem that the detection sensitivity is low. On the other hand, the area of the pressure sensing surface can be made wider when the speed of the fluid is fast. A problem will occur in the detection accuracy if the detection distribution of the pressure sensing surface is not uniform as mentioned above.
The present invention was made under such circumstances.
It is a first object of the present invention to provide a pressure detecting apparatus which can obtain a uniform sensitivity distribution even if a contact type pressure sensor has a pressure sensing surface in which the sensitivity distribution is ununiform.
It is a second object to provide a pressure propagating speed detecting apparatus which can precisely measure the speed and pressure propagating speed of a fluid flow such as blood flow having a low speed and can freely adjust the detection sensitivity.